It has become common in the computing industry to transfer data and commands between data processing devices such as computers, printers, displays, terminals, etc., commonly referred to as "nodes" or network resources. Interconnection of computers and other devices has been achieved via a variety of different networks. One such network is known as a Local Area Network (LAN).
It is predicted that LAN's will be used for many applications in the future. Some of these applications will undoubtedly eliminate the costly and complicated wiring commonly used today. These future applications include automotive, domestic, aviation and ship-borne applications, among others. In these future applications each device may include a specialized unit which provides communication for that device in order to send and receive messages transmitted over the communications medium. In this way, a single medium may be used for multiple purposes such as power transmission to devices connected to a power line as well as communications between these devices connected to the power line. One example of such multiple use network communications is that of a home management system as is described in more detail with reference to FIG. 1. Please note that the same basic issues arise in other situations such as in an office building or storage warehouse.
As can be seen in FIG. 1, a house 100 is shown having a number of devices each coupled via a single communications medium which, in this case, is the standard electrical power line which exists in most houses. These devices can have intelligence so that if a particular device is appropriately signalled it will perform a predefined operation.
Using standard house wiring as the communication medium has the advantage of avoiding the high costs of installing dedicated communication wiring to new or existing buildings. Additionally, using standard house wiring as the communication medium has the advantage of ease of use in that connection to the network is a simple and familiar process for anyone who has ever plugged in a home appliance.
One difficulty in using house wiring for the communication medium is the large amount of noise which exists on the house wiring due to the large variety of pre-existing appliances connected to the wiring network. A further difficulty is caused by all of the attenuation which exists in the house wiring due to a variety of sources as is discussed more fully below. This attenuation of the communications signal may be 40 decibals (dB) or more. Under these conditions it can be very difficult for a communication device to properly receive such an attenuated signal while an adjacent appliance is producing large amounts of noise.